24.1 Structure and Classification of Lipids -...

Prentice Hall © 2003 Chapter Eighteen 1

24.1 Structure and Classification 24.1 Structure and Classification of Lipidsof Lipids

LipidsLipids are naturally occurring molecules are naturally occurring molecules from plants or animals that are from plants or animals that are soluble in soluble in nonpolarnonpolar organic solvents.organic solvents.Lipid molecules contain large hydrocarbon Lipid molecules contain large hydrocarbon portion and not many polar functional group, portion and not many polar functional group, which accounts for their solubility behavior.which accounts for their solubility behavior.


Classification of LipidsClassification of LipidsLipids that are ester or amides of fatty acidsLipids that are ester or amides of fatty acids::

WaxesWaxes –– are carboxylic acid esters where both are carboxylic acid esters where both R groups are long straight hydrocarbon chain. R groups are long straight hydrocarbon chain. Performs external protective functions.Performs external protective functions.TriacylglycerolTriacylglycerol –– are carboxylic acid are carboxylic acid triesterstriesters of of glycerolsglycerols. They are a major source of . They are a major source of biochemical energy.biochemical energy.GlycerophopholipidsGlycerophopholipids -- triesterstriesters of of glycerolsglycerols that that contain charged phosphate contain charged phosphate diestersdiesters. They help . They help to control the flow of molecules into and out of to control the flow of molecules into and out of cells.cells.


SphingomyelinsSphingomyelins –– amides derived from an amides derived from an amino alcohol, also contain charged amino alcohol, also contain charged phosphate phosphate diesterdiester groups. They are essential groups. They are essential to the structure of cell membranes.to the structure of cell membranes.GlycolipidsGlycolipids –– amides derived from amides derived from sphingosinesphingosine, contain polar carbohydrate , contain polar carbohydrate groups. On the cell surface, they connect with groups. On the cell surface, they connect with by intracellular messengers.by intracellular messengers.


Lipids that are not esters or amides:Lipids that are not esters or amides:SteroidsSteroids –– They performs various functions They performs various functions such as hormones and contributes to the such as hormones and contributes to the structure of cell membranes.structure of cell membranes.EicosanoidsEicosanoids –– They are carboxylic acids that They are carboxylic acids that are a special type of intracellular chemical are a special type of intracellular chemical messengers.messengers.


24.3 Properties of Fats and Oils24.3 Properties of Fats and Oils

OilsOils: A mixture of : A mixture of triglycerolstriglycerols that is liquid that is liquid because it contains a high proportions of because it contains a high proportions of unsaturated fatty acids.unsaturated fatty acids.FatsFats: A mixture of : A mixture of triglycerolstriglycerols that is solid that is solid because it contains a high proportions of because it contains a high proportions of saturated fatty acids.saturated fatty acids.


Properties of Properties of triglycerolstriglycerols in natural fats and in natural fats and oils:oils:NonpolarNonpolar and hydrophobicand hydrophobicNo ionic chargesNo ionic chargesSolid Solid triglycerolstriglycerols (Fats) (Fats) -- high proportions of high proportions of saturated fatty acids.saturated fatty acids.Liquid Liquid triglycerolstriglycerols (Oils) (Oils) -- high proportions high proportions of unsaturated fatty acids.of unsaturated fatty acids.


24.4 Chemical Reactions of 24.4 Chemical Reactions of TriglycerolsTriglycerols

HydrogenationHydrogenation: The carbon: The carbon--carbon double bonds carbon double bonds in unsaturated fatty acids can be hydrogenated in unsaturated fatty acids can be hydrogenated by reacting with hydrogen to produce saturated by reacting with hydrogen to produce saturated fatty acids. For example, margarine is produced fatty acids. For example, margarine is produced when two thirds of the double bonds present in when two thirds of the double bonds present in vegetable oil is hydrogenated.vegetable oil is hydrogenated.


Hydrolysis of Hydrolysis of triglycerolstriglycerols: : TriglycerolsTriglycerols like any like any other esters react with water to form their carboxylic other esters react with water to form their carboxylic acid and alcohol acid and alcohol –– a process known as hydrolysis. a process known as hydrolysis.

-- In body, this hydrolysis is catalyzed by the In body, this hydrolysis is catalyzed by the enzyme enzyme hydrolasehydrolase and is the first step in the and is the first step in the digestion of dietary fats and oils.digestion of dietary fats and oils.-- In the laboratory and commercial production In the laboratory and commercial production of soap, hydrolysis of fats and oils is usually of soap, hydrolysis of fats and oils is usually carried out by strong aqueous bases such as carried out by strong aqueous bases such as NaOHNaOH and KOH and is called and KOH and is called saponificationsaponification..


24.5 Cell Membrane Lipids: 24.5 Cell Membrane Lipids: PhosphilipidsPhosphilipids and and GlycolipidsGlycolipids

Cell membranes establish a hydrophobic Cell membranes establish a hydrophobic barrier between the watery environment in the barrier between the watery environment in the cell and outside the cell. Lipids are ideal for cell and outside the cell. Lipids are ideal for this function.this function.The three major kinds of cell membrane lipids The three major kinds of cell membrane lipids in animals are in animals are phospholipids, phospholipids, glycolipidsglycolipids, and , and cholesterolcholesterol. .


Fig 24.4 Membrane lipidsFig 24.4 Membrane lipids


PhosphoilipidsPhosphoilipids contain an ester link between a contain an ester link between a phosphoric acid and an alcohol. The alcohol phosphoric acid and an alcohol. The alcohol is either a glycerol to give a is either a glycerol to give a glycerophopholipidglycerophopholipid or a or a sphingosinesphingosine to give to give sphingomyelinssphingomyelins..GlycolipidsGlycolipids: : GlycolipidsGlycolipids are derived from are derived from sphingosinesphingosine. They differ from . They differ from sphingomyelinssphingomyelins by by having a carbohydrate group at C1 instead of a having a carbohydrate group at C1 instead of a phosphate bonded to a phosphate bonded to a cholinecholine..


24.5 Cell Membrane Lipids: 24.5 Cell Membrane Lipids: CholesterolCholesterol

Animal cell membranes contain significant Animal cell membranes contain significant amount of cholesterol. amount of cholesterol.


Cholesterol is a steroid, a member of the Cholesterol is a steroid, a member of the class of lipids that all contain the same four class of lipids that all contain the same four ring system.ring system.Cholesterol serves two important purposes: Cholesterol serves two important purposes: as a component of cell membranes and as a as a component of cell membranes and as a starting materials for the synthesis of all starting materials for the synthesis of all other steroids.other steroids.


24.7 Structure of Cell Membranes24.7 Structure of Cell MembranesThe basic structural unit of cell membrane is lipid The basic structural unit of cell membrane is lipid bilayerbilayer which is composed of two parallel sheets of which is composed of two parallel sheets of membrane lipid molecules arranged tail to tail. membrane lipid molecules arranged tail to tail. BilayersBilayersare highly ordered and stable, but still flexible.are highly ordered and stable, but still flexible.


Fig 24.7 The cell membraneFig 24.7 The cell membrane


When phospholipids are shaken vigorously When phospholipids are shaken vigorously with water, they spontaneously form with water, they spontaneously form liposome liposome –– small spherical vesicle with lipid small spherical vesicle with lipid bilayerbilayer surrounding an aqueous center. Water surrounding an aqueous center. Water soluble substances can be trapped in the center soluble substances can be trapped in the center of the liposome, and lipidof the liposome, and lipid--soluble substances soluble substances can be incorporated into the can be incorporated into the bilayerbilayer..


24.8 Transport Across Cell 24.8 Transport Across Cell MembranesMembranes

The cell membranes allow the passage of The cell membranes allow the passage of molecules and ions into and out of a cell by two molecules and ions into and out of a cell by two modes; passive transportation and active modes; passive transportation and active transportation.transportation.

Passive transportPassive transport –– substances move across the cell substances move across the cell membrane freely by diffusion from regions of membrane freely by diffusion from regions of higher concentration to regions of lower higher concentration to regions of lower concentration. Glucose is transported into many concentration. Glucose is transported into many cells in this way.cells in this way.


Active transportActive transport -- substances move across the substances move across the cell membrane only when energy is supplied cell membrane only when energy is supplied because they must go in the reverse direction because they must go in the reverse direction from regions of lower to regions of higher from regions of lower to regions of higher concentration. Only by this method, cells concentration. Only by this method, cells maintain lower Namaintain lower Na++ concentration within concentration within cells and higher Nacells and higher Na++ concentration in concentration in extracellularextracellular fluids, with the opposite fluids, with the opposite concentration ratio for Kconcentration ratio for K++..


Fig 24.9 An example of Fig 24.9 An example of active transportactive transport


Properties of cell membranes:Properties of cell membranes:Cell membranes are composed of a fluid like Cell membranes are composed of a fluid like phospholipidphospholipid bilayerbilayer..The The bilayerbilayer incorporates cholesterol, proteins, incorporates cholesterol, proteins, and and glycolipidsglycolipids..Small Small nonpolarnonpolar molecules cross by diffusion molecules cross by diffusion through the lipid through the lipid bilayerbilayer..


Small ions and polar molecules diffuse Small ions and polar molecules diffuse through the aqueous media in protein pores.through the aqueous media in protein pores.Glucose and certain other substances cross Glucose and certain other substances cross with the aid of proteins without energy input.with the aid of proteins without energy input.NaNa++, K, K++, and other substances that maintain , and other substances that maintain concentration gradients inside and outside concentration gradients inside and outside the cell cross with expenditure of energy and the cell cross with expenditure of energy and the aid of proteins.the aid of proteins.


25.2 Lipoproteins for Lipid Transport25.2 Lipoproteins for Lipid Transport

Lipids enter metabolism from three different Lipids enter metabolism from three different sources: (1) the diet, (2) storage in adipose sources: (1) the diet, (2) storage in adipose tissue, and (3) synthesis in the liver. tissue, and (3) synthesis in the liver. Whatever their source these lipids must Whatever their source these lipids must eventually be transported in blood, an aqueous eventually be transported in blood, an aqueous media. media. To become water soluble, fatty acid release To become water soluble, fatty acid release from adipose tissue associate with albumin. All from adipose tissue associate with albumin. All other lipids are carried by lipoproteins.other lipids are carried by lipoproteins.


Fig 25.5 Transport of lipidsFig 25.5 Transport of lipids

24.1 Structure and Classification of Lipids -...

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